A rotary encoder, a rotary pulse generator, and the like are known as devices for measuring rotational angles, rotational positions, and the rotating speed of rotors.
A rotary encoder converts an analog measurement of rotational angle to a corresponding digital value by generating pulse signals with frequencies and amplitudes proportional to the analog measurement. The pulse signals can be used to measure a length along which a plate of stainless steel is cut, to detect a displacement of an arm or body of a robot, to choose tools for a machining center through a measurement of rotational angles, etc. The pulse signals also can be used to detect the speed of cars and the engines thereof, to control rotational angles of dc-motors which are used in tape recorders, facsimile machines, printers, and the like. Rotary pulse generators operate in the same manner as rotary encoders, and are preferably used for detecting the rotational angle of low speed rotors.
A general scheme of a conventional rotational angle detecting device is shown in FIG. 1. Reference numeral 4 designates a motor the rotational angle of which is to be detected. A rotating disk 3 is attached to a shaft 5 of the motor 4.
A top view of the rotating disk 3 is shown in FIG. 2. Photowindows 6 are formed to have a constant pitch along a circumferential direction and to allow light to pass through them. A light source 1, such as a light emission diode, and a photodetector 2, such as a photodiode, are opposed to each other. The rotating disk 3 is placed therebetween.
The light passing through the photowindows 6 reaches the photodetector 2 intermittently as the disk 3 rotates. Consequently, the photodetector 2 generates an output current 7 having a sinusoidal waveform as shown in FIG. 3(a). The current 7 is shaped to be a pulse train 8 as shown in FIG. 3(b) through a device such as a Schmitt trigger circuit (not shown in the figure). The pulse train can be used for operation and control.
If the number of the photowindows 6 is No, the number of rotations after the start of the motor 4 is m, and the number of pulses generated in this period is N, the following relation is obtained; EQU N/No=m+(N-mNo)/No (1)
The value of 360.times.(N-mNo) designates an angular difference between a reference point of the motor 4 and that of the stationary plate for counting the number of rotations of the motor 4. Therefore, the accuracy for measuring the angular difference is expected to be 360/N and it is necessary to make No as large as possible in order to improve the accuracy of the device.
As for the sensitivities of the photodiodes and the solar batteries conventionally used as the photodetector 2, an output current density of only about 10-20 uA/cm.sup.2 is obtained under the illuminance of 100 lux, while an output current of at least 1 microamperes is required to achieve ready display and control at a low cost. Therefore, a lower limit for the size of the photowindows 6 exists from a viewpoint of utility.
If an evaluation is carried out by assuming that conventionally available photoreceivers are used from a viewpoint of utility, areas of 3 mm.times.3 mm are required to obtain an output current of about 1 microamperes. Accordingly, for a rotating disk with a diameter of about 40 mm, at least fifteen photowindows are required. In this case the measuring accuracy of the angular difference is 24 degrees.
When an output current of 1 microamperes is obtained, it is usual that the output current would be converted to a voltage drop of about 10 millivolts through a resistance of about 10 KOhms, and be further amplified to about one hundred times larger through a conventional amplifier. For a conventional rotating disk which can generate 6000 pulses per revolution, the output current of the photodetector is 1 microamperes.times.1/400 (=15/6000). Accordingly further amplification of the output current of 400 times is required. This high amplification causes distortions and makes it difficult to achieve stable amplification. Also, the amplifier circuit becomes more expensive.
Moreover, if the shaft 5 is attached to a point deviated from the center of the disk 3, the amount of light passing through the photowindows 6 will fluctuate. Pulsations will appear in the output current of FIG. 3(a) with a period equal to the cycle of rotations of the disk 3. The pulsations cause distortion of the waveform generated by the reforming circuit, which in turn degrades the accuracy of detection of the rotational angle.
The rotary disk 3 is usually constructed by making slits to be used as the photowindows, or by printing patterns of opaque material on a transparent glass disk such that the photowindows are located where the patterns do not exist. Irrespective of the processing methods, variations in the shapes, sizes, and positions of the photo windows cannot be avoided. The accuracy of detection of the rotational angle is adversely affected by the variations in the photowindows, since they cause distortions in the output current waveform from the photodetector. The phase and shape of individual pulses in the pulse train will fluctuate.